This invention relates to a method of determining the offset of the head part of an orthopaedic joint prosthesis stem component, relative to the neck of the stem part on which the head part can be fitted, and to a template kit for use in such a method.
Modular stem components of an orthopaedic joint prosthesis can comprise a stem part which can be fitted into a prepared cavity in a long bone, and a head part which has an articulating surface for articulation with the opposed bearing surface patient's anatomic joint, or with the bearing surface of another component of the joint prosthesis. Such prostheses can be used in, for example, the replacement of hip joints and of shoulder joints.
The stem component can extend into the medullary canal of the bone (which might be the femur or the humerus). The stem component will define a neck axis. The neck is directed towards the other component of the joint prosthesis when the prosthesis is implanted. The head part of the prosthesis is fitted on to the neck of the stem part. The stem part can have an intramedullary portion and a neck portion. As is known, the angle between the intramedullary and neck portions is usually between about 30° and about 50°, to match the anatomy of the patient. For some applications, the stem component can have a neck portion which sits in the neck region of the patient's bone, without an intramedullary portion.
The head part can have a bearing surface which provides for articulation with a corresponding bearing surface of the mating component of the joint prosthesis. The bearing surface of the head part can be convex, for articulation with a concave bearing surface of the mating component. This will frequently be the arrangement in the case of a hip joint prosthesis and can also be the arrangement in the case of a shoulder joint prosthesis. The bearing surface of the head part can be concave, for articulation with a convex bearing surface of the mating component, as can be the arrangement in the case of shoulder joint prostheses, sometimes referred to as reversed shoulder joint prostheses.
It can be preferred for the axis of the head part to be offset relative to the axis of the neck of the stem part of a joint prosthesis component. This can facilitate matching the configuration of the component to the patient's anatomy. Varying the offset in the plane which is perpendicular to the anterior-posterior axis can affect the effective length of the patient's limb. Varying the offset in the plane which is perpendicular to the medial-lateral axis can affect the degree of retroversion or anteversion in the replacement joint.